The Official Plan Flood (OPF) is the flood on which all Miami Conservancy District channels, levees and dams are designed. The March 1913 flood was, and still is, the flood of record for the Great Miami River Watershed. The rain that fell on already frozen ground had a runoff rate of 90 percent. 

After an exhaustive investigation of precipitation and flooding data, Arthur Morgan, the system’s engineer, felt that a flood protection system that could handle the 1913 flood, plus 40 percent additional runoff, should protect the cities from all flooding by the Great Miami River.